1. Field of the Invention
The present invention pertains to methods for treating cellular proliferative disorders in a mammal that are mediated by Ras-activation using mutant viruses.
2. References
The following publications, patent applications and patents are cited in this application:    1. Beattie, E. et al., Virology (1991) 183:419-422    2. Black, T. L., et al., J. Virol. (1993) 67:791-800    3. Chang, H. W. and Jacobs, B. L. Virology (1993) 194:537-547    4. Chong, K. L. et al., EMBO J. (1992) 11:1553-1562    5. Davies, M. V. et al., JBC (1991) 266:14714-14720    6. Davies, M. V. et al., J. Virology (1993) 67:1688-1692    7. Jagus, R. and Gray M. M. Biochimie (1994) 76:779-791    8. Katze, M. G. et al., EMBO J (1987) 6:689-697    9. Katze M. G. et al., Trends in Microbiology (1995) 3:75-78    10. Lee, T. G. et al., MCB (1994) 14:2331-2342    11. Mundshau, L. J. and Faller, D. V., JBC (1992) 267:23092-23098    12. Mundshau, L. J. and Faller, D. V., Biochimie (1994) 76:792-800    13. Nanduri, S. EMBO J. (1998) 17:5458-5465    14. Proud, D. G. Trends in Biochemical Sciences, (1995) 20:241-246    15. Redpath, N. T. and Proud, D. G. Biochimica et Biophysica Acta, (1994) 1220:147-162    16. Strong, J. E. et al., EMBO (1998) 17:3351-3362    17. Williams, B. R., Biochemical Society Transactions (1997) 25:509-513    18. Wiessmuller, L. and Wittinghofer, F., Cellular Signaling (1994) 6(3):247-267    19. Barbacid, M., A Rev. Biochem. (1987) 56:779-827    20. Millis, N. E. et al., Cancer Res. (1995) 55:1444    21. Chaubert, P. et al., Am. J Path. (1994) 144:767    22. Bos, J., Cancer Res. (1989) 49:4682    23. Levitzki A., Eur. J. Biochem. (1994) 226:1    24. James P. W., et al., (1994) Oncogene 9:3601    25. Lee J. M. et al., PNAS (1993) 90:5742-5746    26. Lowe S. W. et al., Science, (1994) 266:807-810    27. Raybaud-Diogene H. et al. J. Clin. Oncology, (1997) 15(3): 1030-1038    28. Brooks et al., eds. “Jawetz, Melnick, & Adelberg's Medical Microbiology,” (1998)    29. He, B. et al, PNAS (1997) 94:843-848    30. Haig, D. M. et al., Immunology (1998) 93:335-340    31. Kawagishi-Kobayashi, M. et al., MCB (1997) 17:4146-4158    32. Martuza et al., European Patent Application Publication Number EP 0 514 603, published Nov. 25, 1992
All of the above publications, patent applications and patents are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent application or patent was specifically and individually indicated to be incorporated by reference in its entirety.
3. State of the Art
Normal cell proliferation is regulated by a balance between growth promoting proto-oncogenes and growth-constraining tumor-suppressor genes. Tumorigenesis can be caused by genetic alterations to the genome that result in the mutation of those cellular elements that govern the interpretation of cellular signals, such as potentiation of proto-oncogene activity or inactivation of tumor suppression. It is believed that the interpretation of these signals ultimately influences the growth and differentiation of a cell, and that misinterpretation of these signals can result in neoplastic growth (neoplasia).
Genetic alteration of the proto-oncogene Ras is believed to contribute to approximately 30% of all human tumors.18, 19 The role that Ras plays in the pathogenesis of human tumors is specific to the type of tumor. Activating mutations in Ras itself are found in most types of human malignancies, and are highly represented in pancreatic cancer (80%), sporadic colorectal carcinomas (40-50%), human lung adenocarcinomas (15-24%), thyroid tumors (50%) and myeloid leukemia (30%).20, 21, 22 Ras activation is also demonstrated by upstream mitogenic signaling elements, notably by tyrosine receptor kinases (RTKs). These upstream elements, if amplified or overexpressed, ultimately result in elevated Ras activity by the signal transduction activity of Ras. Examples of this include overexpression of PDGFR in certain forms of glioblastomas, as well as in c-erbB-2/neu in breast cancer.21,23,24 
Protein kinase R (“PKR”) is a serine/threonine kinase that is induced in the presence of interferon.7, 9, 17 The primary cellular substrate of this kinase is the α subunit of the translation initiation factor eIF-2 on Serine 5.14,15,17 Phosphorylation of eIF-2 results in a rapid inhibition of protein synthesis by preventing its participation in further rounds of translation initiation.
Although PKR is normally inactive, it becomes rapidly activated in the presence of double stranded RNA (dsRNA) or RNAs that exhibit extensive secondary structures, elements that are frequently produced as the result of viral infection. The amino-terminal of PKR contains a double stranded RNA binding domain (dsRBD) that allows this interaction with dsRNA. Binding of PKR to dsRNA element allows PKR to undergo a conformational change that facilitates autophosphorylation and subsequent phosphorylation of eIF-2.4 Further, it appears that the cooperative binding of two PKR molecules to one dsRNA molecule is required to achieve activation since the addition of dsRNA to PKR results in the dsRNA/PKR activation complex to be found in a 2:1 ratio of protein to dsRNA.17 
Double-stranded RNA (dsRNA) viruses are not entirely susceptible to the host cell PKR because they have evolved a number of different strategies to inhibit. PKR activation in response to their presence:
(1) In the case of adenovirus, a viral product, VAI RNA, is synthesized in large amounts. These VAI RNA elements, with their extensive secondary structure and short length inactivate PKR by acting as a competitive inhibitor of the full length viral dsRNA.8 The short length of the VAI RNA elements is critical, as there is a minimum length dsRNA which activates PKR. PKR bound to VAI RNA is not activated;
(2) Vaccinia virus encodes two gene products, K3L and E3L to down-regulate PKR with different mechanisms. The K3L gene product has limited homology with the N-terminal region of eIF-2α and may act as a pseudosubstrate for PKR.1,5 The E3L gene product is a dsRNA-binding protein and apparently functions by sequestering activator dsRNAs;3, 6 
(3) Herpes simplex virus (HSV) gene γ134.5 encodes the gene product infected-cell protein 34.5 (ICP34.5) that can prevent the antiviral effects exerted by PKR; and
(4) The parapoxvirus orf virus encodes the gene OV20.0L that is involved in blocking PKR activity.30 
It has been demonstrated that in Ras transformed cells, dsRNA-mediated activation of PKR was blocked at the level of autophosphohrylation.16 
PKR is one of many cellular proteins that is induced in the presence of interferon (“IFN”). In normal cells, PKR is normally induced and activated in the presence of IFN. In Ras-mediated tumor cells, however, PKR is induced in the presence of IFN but the activation of PKR is reversed or inhibited. Accordingly, Ras-mediated tumors are unable to activate a PKR response.
It has been observed that pre-treating cells with IFN to induce the transcription and translation of PKR prevents reovirus infection. PKR was activated in cells that were pre-treated with IFN, suggesting that there may be a “quantity effect.” When the cells were not pre-treated with IFN, reovirus was able to replicate quickly enough such that there was not enough time to allow sufficient PKR to be synthesized. Additionally, the PKR already present in the cell was not activated. This observation suggests that the cells are not deficient in the IFN response per se, since PKR is only one element of the IFN response and PKR apparently acted normally if the cells were pre-treated.
Current methods of treatment for neoplasia include surgery, chemotherapy and radiation. Surgery is typically used as the primary treatment for early stages of cancer; however, many tumors cannot be completely removed by surgical means. In addition, metastatic growth of neoplasms may prevent complete cure of cancer by surgery. Chemotherapy involves administration of compounds having antitumor activity, such as alkylating agents, antimetabolites, and antitumor antibiotics. The efficacy of chemotherapy is often limited by severe side effects, including nausea and vomiting, bone marrow depression, renal damage, and central nervous system depression. Radiation therapy relies on the greater ability of normal cells, in contrast with neoplastic cells, to repair themselves after treatment with radiation. Radiotherapy cannot be used to treat many neoplasms, however, because of the sensitivity of tissue surrounding the tumor. In addition, certain tumors have demonstrated resistance to radiotherapy and such may be dependent on oncogene or anti-oncogene status of the cell.25, 26, 27 Martuza et al., EP 0 514 60332, generically describes methods for selectively killing neoplastic cells which utilize altered viruses that are capable of replication in neoplastic cells while sparing surrounding normal tissue.
Accordingly, it has been found that viruses which have evolved certain mechanisms of preventing PKR activation are likely rendered replication incompetent when these same mechanisms are prevented or mutated. Mutation or deletion of the genes responsible for antagonizing PKR should prevent viral replication in cells in which the PKR activity is normal (i.e. normal cells). However, if infected cells are unable to activate the antiviral response mediated through PKR (i.e., Ras-mediated tumor cells), then these mutant viruses should replicate unheeded and cause cell death. Therefore, these mutant viruses can replicate preferentially in Ras-transformed cells where it is determined that PKR is unable to function.
In view of the drawbacks associated with the current means for treating neoplastic growth, the need still exists for improved methods for the treatment of most types of cancers.